British Patent Application No. 2081429A discloses a firearm for discharge of rounds such as gas canisters and the like. The firearm comprises a body in which there is a cavity for a round of ammunition with a percussion detonating means in a rearward facing surface thereof, at least one transverse aperture for introduction of a round into the cavity and ejection of the round therefrom, a breech block rearward of the cavity and movable to and from a rest disposition to a firing disposition forward of the rest disposition to carry the front of the round forwardly into engagement with a seating at the forward end of the cavity, and a firing pin carried by the breech block with a forward end to impact with the detonating means for detonation and discharge of the round, and a trigger to actuate the movement of the breech block and the firing pin, the firing mechanism comprising means to transmit the trigger movement to the breech block and lock the breech block in the firing disposition.
The prior published application discloses, in particular, an actuating mechanism for a firearm of this type in which use is made of an over-centre toggle linkage to lock the breech block in the firing disposition, and a sear mounted in the body of the firearm for holding and release of the firing pin upon movement of the trigger.